Patent GB-929631-A (GB)
Title: Traffic monitoring system
IPC: G08G1/01
Filed: 1959-04-30
Abstract: 929,631. Electric analogue computers. LABORATORY FOR ELECTRONICS Inc. April 30, 1959 [May 1, 1958], No. 14835/59. Class 37. [Also in Groups XXXVIII and XL (c)] An apparatus for approximating a running average of a last predetermined number of a continuing series of individual random values comprises means for receiving in succession electrical values representative of the random values and for deriving and storing a first electrical value representative of the average of the electrical values received prior to the latest of the electrical values, means for storing the latest value, and means for adjusting the stored derived value by adding algebraically thereto a fraction of the difference between the latest value and the stored first value, the fraction being obtained by dividing the difference by the predetermined number. The invention is described with reference to a traffic monitoring arrangement in which facilities are provided for: Obtaining running average of the speed or R.M.S. speed of the last N vehicles. Indicating or recording the speed. Indicating or recording the volume of traffic. Classifying the speeds or volumes of traffic and recording the total time that the speed or volume is in the various classes. Antennae 11, Fig. 1(a), of radar sensing unit 10 working on 2455 megacycles is mounted at an angle of 35 degrees with the road to produce a horizontally polarised beam having a transverse width of 20 degrees between half power points and a beam width between TES and LES, Fig. 3b, in the vertical plane of 60 degrees. Four dipoles in a horizontal row have been found suitable and produce an elliptical pattern on the road extending up to 200 feet. The approach of a vehicle is detected in zero 0, Fig. 3a, and the Doppler frequency which is the difference between the cavity resonance frequency and the reflected wave, and appears as an output from grid 22 of a resonant cavity oscillator 2, is measured to provide the speed indication. The oscillator and amplifiers including Doppler frequency band-pass filters 8-1000 cs/s. providing an output to a line transformer for conveyance over telephone facilities or radio to a remote point, are described with reference to Fig. 2 (not shown). The arrangement includes sensitivity controls, line length compensation arrangements and test jacks. The signal received at the monitoring centre is applied to an impulse detector circuit, Fig. 4b, housing a pass-band 12-17 c/s., to actuate a vehicle detection relay 170. The detector circuit includes squaring amplifiers 135, 136 which produce square waves at Doppler frequency which are clipped +VE ly and -VE ly respectively by rectifiers 132, 135, and added in double triode 152 to give a D.C. relay actuating output which is smoothed, by capacitors 166, 167, to prevent false actuation by reflection from a number of vehicles. Speed detector circuit, Fig. 4a, to which the Doppler signal is simultaneously applied via transformer 51 has a pass-band of 30-750 cycles and the signal after amplification is limited in amplifier 61 to prevent saturation by large input signals, and applied to a counter 64 which produces a D.C. output proportional to frequency across cathode resistor 65. An expander circuit 101 operated from the limiter prevents response to extraneous noise signals e.g. due to traffic in adjacent lanes, and effects cut-off of the response from the vehicle whose speed is being measured when the response falls below a predetermined level. Outputs are provided from tappings 80, 81 on output cathode followers 71, 72 to the speed averaging computer, a panel speed indicator 86 and a speed recorder. Linearity is achieved by voltage stabilizing arrangements of tubes 90, 92 and triodes 83, 84. Speed signals are converted from D.C. to A.C. in an amplifier, Fig. 7c (not shown) and applied under the control of relay arrangement, Fig. 7d, and a servomotor 271, Fig. 7b, to set a wiper on a last car potentiometer 267a to a position corresponding to speed, the speed as set being indicated on a meter 270. This is achieved by feeding the input signal, received on a condenser 263, to the grid 255 of a cathode follower 258, and the output J, from potentiometer 267a, to the grid 256 of cathode follower 259, and using the difference, chopped and amplified, to produce a signal having a phase relative to a standard dependent on the difference to energize the motor field until the difference is eliminated, and the new car speed is set in potentiometer 217, a null circuit 277a, 280, 281 operating relay A1N at balance. Wiper 283 on potentiometer 282 which is ganged to wiper 267 produces an output proportional to the last car speed   squared on terminal R. and 1/nth of the difference between the last car speed squared and the average car speed [for N vehicles] squared, as set up on average car speed squared potentiometer 291 is added by the operation of relay Ab to the average car speed squared on a capacitor 293. The total voltage on capacitor 293 is applied to the grid 255 of cathode follower 258, and the average car speed squared [for N vehicles] from pot. 291 is applied to grid 256 of cathode follower 259, the wiper on average car speed potentiometer 291 being reset by any difference to the new average speed by servomotor 307. The output from potentiometer 290a gives the R.M.S. value. A higher frequency is injected into the servomotors to overcome friction, a negative supply 248 is provided to prevent back drive beyond zero, and the number of cars of which the average is taken is made adjustable by a potentiometer 289. Manual testing facilities are also provided. A traffic volume measuring arrangement which is operated also by impulses from the relay 170 and includes a capacitor charging circuit in circuit with a transformer primary to produce a square wave for feeding a traffic volume computer is described with reference to Fig. 5 (not shown), the indication being in quantity over a given time interval, or as a percentage of a known traffic volume. Classification of the traffic volume, or speed average is arranged by a group of relays each of which is operable at a different voltage level to control a time meter and is described with reference to Fig. 6 (not shown). Switchable arrangements are included in the servomotor circuit to respond to and indicate speed and average speed or R.M.S. speeds and R.M.S. averages.